Foamable composition and process for making large cell size alkenyl aromatic polymer foam structure with 1,1-difluoroethane

ABSTRACT

Disclosed is a process for making extruded, closed-cell alkenyl aromatic polymer foam structure having an average cell size of from about 0.3 to about 3.0 millimeters. The process comprises: a) heating an alkenyl aromatic polymer material having greater than 50 percent by weight alkenyl aromatic monomeric units to form a melt polymer material; b) incorporating into the melt polymer material at an elevated pressure a blowing agent comprising at least 50 percent by weight 1,1-difluoroethane (HFC-152a) based upon the total weight of the blowing agent to form a foamable gel; c) cooling the foamable gel to an optimum foaming temperature prior to extrusion through a die; and d) extruding the foamable gel through a die to form the foam structure. Further disclosed is a foamable gel capable of forming an alkenyl aromatic polymer foam structure having an average cell size of from about 0.3 to about 3.0 millimeters.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 07/927,110 filed Aug. 7,1992.

BACKGROUND OF THE INVENTION

This invention relates to a process for making a large cell size alkenylaromatic polymer foam structure with 1,1-difluoroethane. The inventionfurther relates to a foamable gel capable of being expanded to form sucha foam structure.

Alkenyl aromatic polymer foam structures have been used extensivelycommercially in floral and craft applications. The most desirable ofsuch foam structures are typically closed-cell, and further are ofrelatively large cell size, i.e. 0.3 to 3.0 millimeters.

Due to environmental concerns, it would be desirable to blow large cellsize alkenyl aromatic polymer foam structures with a blowing agenthaving a low ozone depletion potential. One such blowing agent is1,1-difluoroethane (HFC-152a). HFC-152a is relatively economical, and isreadily-available commercially.

In making an alkenyl aromatic polymer foam structure of relatively largecell size, one skilled in the art might select anenvironmentally-acceptable blowing agent having relatively highsolubility in the alkenyl aromatic polymer. High solubility would besought after because, as a general principle, blowing agents withrelatively high solubility in a particular polymer usually produce foamstructures with relatively large cell sizes. Blowing agents ofrelatively low solubility usually produce foam structures havingrelatively small cell size. Thus, it would not be expected that ablowing agent of relatively low solubility, such as HFC-152a, in analkenyl aromatic polymer would produce a foam structure of relativelylarge cell size.

Surprisingly, it has been found that a large cell alkenyl aromaticpolymer foam structure can be blown using HFC-152a as a primary blowingagent. Further, the foam structure blown with HFC-152a has high heatdistortion temperature properties and low impact strengthcharacteristics.

SUMMARY OF THE INVENTION

According to the present invention, there is a process for makingextruded, closed-cell alkenyl aromatic polymer foam structure having anaverage cell size of from about 0.3 to about 3.0 millimeters. Theprocess comprises: a) heating an alkenyl aromatic polymer materialhaving greater than 50 percent by weight alkenyl aromatic monomericunits to form a melt polymer material; b) incorporating into the meltpolymer material at an elevated pressure a blowing agent comprising atleast 50 percent by weight 1,1-difluoroethane (HFC-152a) based upon thetotal weight of the blowing agent and a second blowing agent of lowervapor pressure and higher solubility in the melt polymer material than1,1-difluoroethane to form a foamable gel; c) cooling the foamable gelto an optimum foaming temperature prior to extrusion through a die; andd) extruding the foamable gel through a die to form the foam structure.The second blowing agent is preferably selected from ethyl chloride,ethanol, acetone, methanol, propanol, dimethyl ether, and ethyl acetate.Ethyl chloride is most preferred.

Further according to the present invention, there is a foamable gelcapable of being expanded to form an alkenyl aromatic polymer structurehaving an average cell size of from about 0.3 to about 3.0 millimeters.The gel has the blowing agents in the proportions described above.

DETAILED DESCRIPTION

The foam structure is comprised of an alkenyl aromatic polymer material.Suitable alkenyl aromatic polymer materials include alkenyl aromatichomopolymers and copolymers of alkenyl aromatic compounds andcopolymerizable ethylenically unsaturated comonomers. The alkenylaromatic polymer material may further include minor proportions ofnon-alkenyl aromatic polymers. The alkenyl aromatic polymer material maybe comprised solely of one or more alkenyl aromatic homopolymers, one ormore alkenyl aromatic copolymers, a blend of one or more of each ofalkenyl aromatic homopolymers and copolymers, or blends of any of theforegoing with a non-alkenyl aromatic polymer. Regardless ofcomposition, the alkenyl aromatic polymer material comprises at greaterthan 50 and preferably at greater than 90 weight percent of alkenylaromatic monomeric units. Most preferably, the alkenyl aromatic polymermaterial is comprised entirely of alkenyl aromatic monomeric units.

Suitable alkenyl aromatic polymers include those derived from alkenylaromatic compounds such as styrene, alphamethylstyrene, ethylstyrene,vinyl benzene, vinyl toluene, chlorostyrene, and bromostyrene. Apreferred alkenyl aromatic polymer is polystyrene. Minor amounts ofmonoethylenically unsaturated compounds such as C₁₋₄ alkyl acids andesters, ionomeric derivatives, and C₂₋₆ dienes may be copolymerized withalkenyl aromatic compounds. Examples of copolymerizable compoundsinclude acrylic acid, methacrylic acid, ethacrylic acid, maleic acid,itaconic acid, acrylonitrile, maleic anhydride, methyl acrylate, ethylacrylate, methyl methacrylate, vinyl acetate and butadiene. Preferredstructures comprise substantially (i.e. greater than 95%) and mostpreferably entirely of polystyrene because polystyrene foam iseconomical, and is commonly employed as an insulating plastic foam.

According to the present invention, the alkenyl aromatic polymer foam isgenerally prepared by heating an alkenyl aromatic polymer material toform a plasticized or melt polymer material, incorporating therein ablowing agent to form a foamable gel, and extruding the gel through adie to form the foam product. Prior to mixing with the blowing agent,the polymer material is heated to a temperature at or above its glasstransition temperature or melting point. The blowing agent may beincorporated or mixed into the melt polymer material by any means knownin the art such as with an extruder, mixer, blender, or the like. Theblowing agent is mixed with the melt polymer material at an elevatedpressure sufficient to prevent substantial expansion of the melt polymermaterial and to generally disperse the blowing agent homogeneouslytherein. Optionally, a nucleator may be blended in the polymer melt ordry blended with the polymer material prior to plasticizing or melting.The foamable gel is typically cooled to a lower temperature to optimizephysical characteristics of the foam structure. The gel is then extrudedthrough a die of desired shape to a zone of lower pressure to form thefoam structure.

The primary component of the blowing agent to make the present structureis 1,1-difluoroethane (HFC-152a). It comprises greater than 50 percentto about 90 weight percent of the blowing agent based upon the totalweight of the blowing agent.

The blowing agent further comprises a secondary blowing agent presentfrom between about 10 to less than 50 weight percent and preferably fromabout 20 to about 40 weight percent based upon the total weight of theblowing agent. The secondary blowing agent will have a lower vaporpressure in air at 25° C. than HFC-152a. The secondary blowing agentwill further be more soluble in the alkenyl aromatic polymer thanHFC-152a. The secondary blowing agent will preferably have a vaporpressure in air at 25° C. of less than 580 kilopascals, and preferablyhave a solubility in polystyrene (200,000 weight average molecularweight according to size exclusion chromatography) of greater than 1.9parts per hundred by weight at 25° C. per atmosphere of air pressurebased upon the weight of the polymer. Preferred secondary blowing agentsare ethyl chloride, ethanol, acetone, methanol, propanol, dimethylether, and ethyl acetate. Ethyl chloride is most preferred.

The blowing agent may further comprise small amounts (less than 15weight percent) of a tertiary blowing agent comprising other knownblowing agents including inorganic agents, organic blowing agents otherthan those mentioned above, and chemical blowing agents. Suitableinorganic blowing agents include carbon dioxide, nitrogen, argon, water,air, and helium. Organic blowing agents include aliphatic hydrocarbonshaving 1-9 carbon atoms and fully and partially halogenated aliphatichydrocarbons having 1-4 carbon atoms. Aliphatic hydrocarbons includemethane, ethane, propane, n-butane, isobutane, n-pentane, isopentane,neopentane, and the like. Fully and partially halogenated aliphatichydrocarbons include fluorocarbons, chlorocarbons, andchlorofluorocarbons. Examples of fluorocarbons include methyl fluoride,perfluoromethane, difluoromethane (HFC-32), ethyl fluoride,1,1,1-trifluoroethane (HFC-143a), 1,1,1,2-tetrafluoro-ethane (HFC-134a),pentafluoroethane, perfluoroethane, 2,2-difluoropropane,1,1,1-trifluoropropane, perfluoropropane, perfluorobutane,perfluorocyclobutane. Partially halogenated chlorocarbons andchlorofluorocarbons for use in this invention include methyl chloride,methylene chloride, 1,1,1-trichloroethane, 1,1-dichloro-1-fluoroethane(HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b),1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124). Fully halogenatedchlorofluorocarbons include trichloromonofluoromethane (CFC-11 ),dichlorodifluoromethane (CFC-12), trichlorotrifluoroethane (CFC-113),dichlorotetrafluoroethane (CFC-114), chloroheptafluoropropane, anddichlorohexafluoropropane. Chemical blowing agents includeazodicarbonamide, azodiisobutyro-nitrile, benzenesulfonhydrazide,4,4-oxybenzene sulfonyl-semicarbazide, p-toluene sulfonylsemi-carbazide, barium azodicarboxylate,N,N'-dimethyl-N,N'-dinitrosoterephthalamide, and trihydrazino triazine.

The amount of blowing agent incorporated into the polymer melt materialto make a foam-forming polymer gel is from about 0.2 to about 5.0,preferably from about 0.5 to about 3.0, and most preferably from about1.0 to 2.50 moles per kilogram of polymer.

A surprising feature of this invention is that it is possible to blow aclosed-cell, alkenyl aromatic polymer foam structure using HFC-152a asthe primary blowing agent. The use is surprising in view of itsrelatively low solubility in alkenyl aromatic polymers, such aspolystyrene, and its relatively high vapor pressure. Typically, theability of a blowing agent to produce a foam structure with relativelylarge cells has been observed to decrease as its solubility in thepolymer decreases and as its vapor pressure increases. Thus, a blowingagent with relatively low solubility and a relatively high vaporpressure will usually produce a relatively small cell size foamstructure. A blowing agent with relatively high solubility andrelatively low vapor pressure will usually produce a relatively largecell size foam structure. The process of the present invention issurprising because it does not follow previous observations.

Table 1 illustrates solubility and vapor pressure data for severalcommon blowing agents.

                                      TABLE 1                                     __________________________________________________________________________    SOLUBILITY AND VAPOR PRESSURE DATA                                            FOR COMMON BLOWING AGENTS                                                            Ethyl                                                                         Chloride                                                                           Ethanol                                                                            HCFC-142b                                                                            CFC-12                                                                             HCFC-22                                                                             HFC-152a                                                                            HFC-134a                             __________________________________________________________________________    Solubility*                                                                          18.0 22.2 6.3    1.5  1.6   1.8   1.0                                  Vapor  163.4.sup.a                                                                        394.0.sup.a                                                                        337.9.sup.b                                                                          651.3.sup.a                                                                        1044.sup.b                                                                          598.5.sup.b                                                                         665.4.sup.b                          Pressure                                                                      (25° C.) kPa                                                           __________________________________________________________________________     *pph/atm in polystyrene                                                       .sup.a Chemical Engineers Handbook, 5th ed.                                   .sup.b DuPont Brochure AG1 "Alternatives to                                   HCFC142b is 1chloro-1,1-difluoroethane                                        CFC12 is dichlorodifluoromethane                                              HCFC22 is chlorodifluoromethane                                               HFC152a is 1,1difluoroethane                                                  HFC134a is 1,1,1,2tetrafluoroethane                                      

Relatively large cell size alkenyl aromatic polymer foams have been madeusing HCFC-142b with or without ethyl chloride. HCFC-142b has been usedsuccessfully in making large cell size foams because of its relativelymoderate solubility in alkenyl aromatic polymers and its relativelymoderate vapor pressure.

Other above-mentioned blowing agents, namely CFC-12, HCFC-22, HFC-134a,typically have not been successfully employed in making relatively largecell foams due to their relatively low solubility in alkenyl aromaticpolymers and high vapor pressure. Given that HFC-152a has similar vaporpressure and solubility in alkenyl aromatic polymers as those blowingagents, it is surprising that a relatively large cell size foam could beproduced with it.

Ethyl chloride and ethanol have not been used successfully as primaryblowing agents because of foam structure collapse problems, but havebeen used successfully as secondary blowing agents.

The foam component of the foam structure has the density of from about10 to about 150 and most preferably from about 10 to about 70 kilogramsper cubic meter (kg/m³). The foam has an average cell size of from about0.3 to about 3.0 and preferably from about 0.4 to about 2.0 millimeters(mm) according to ASTM D3576.

The foam component of the present foam structure is closed-cell.Preferably, the present foam is greater than 90 percent closed-cellaccording to ASTM D2856-A.

Various additives may be incorporated in the present foam structure suchas inorganic fillers, pigments, antioxidants, acid scavengers,ultraviolet absorbers, flame retardants, processing aids, extrusionaids, and the like.

The foam structure is most desirably used as a substrate in floral andcraft, buoyancy and flotation billet, and sign display boardapplications. The foam structure is readily machinable or fabricable toform a variety of shapes.

The foam structure may be used to insulate a surface by applying to thesurface an insulating panel fashioned from the present structure. Suchpanels are useful in any conventional insulating applications such asroofing, buildings, refrigerators, etc.

The foam structure may be formed into a plurality of discrete foamedparticles for conventional loose-fill cushioning and packagingapplications, or may be ground into scrap for use as blown insulation.

Though the preferred process for making the present structure is anextrusion process, it is understood that the above structure may beformed by expansion of beads, which may be molded at the time ofexpansion to form structures of various shapes. Insulating panels formedfrom molded, expandable beads are commonly referred to as bead board.

The following are examples of the present invention, and are not to beconstrued as limiting. Unless otherwise indicated, all percentages,parts, or proportions are by weight.

EXAMPLE(S)

Foam structures were prepared according to the process of the presentinvention using HFC-152a as the primary blowing agent and ethyl chlorideor ethanol as the secondary blowing agent. The foam structures producedhad the desired large average cell sizes.

In each example, an 21/2" (64 mm) extruder operated at 90 kg/hrpolystyrene. The alkenyl aromatic polymer employed was polystyrene witha 200,000 weight average molecular weight as measured by size exclusionchromatography. All additives were dry blended with the polystyrene, andfed to the extruder. Blowing agents were incorporated into the polymermelt by means of a separate mixer. Cell size was measured according toASTM D3576. All weight proportions, indicated as parts per hundred(pph), were based upon the weight of the polystyrene.

EXAMPLE 1

Additives were: hexabromocyclododecane (HBCD) at 1.0 pph, bariumstearate at 0.05 pph, tetrasodium pyrophosphate at 0.02 pph, and linearlow density polyethylene (LLDPE) at 0.30 pph. The blowing agent was amixture of 6.6 pph HFC-152a and 4.0 pph ethyl chloride. The die gap was2.9 mm and the die pressure was 4240 kilopascals (kPa).

The foam structure produced had a cell size of 1.3 mm, a desirable largeaverage cell size. The structure further had a density of 27.9 kg/m³,and a compressive strength balance (CSB) of 1.3. CSB is a ratio of thehighest compressive strength measurement to the lowest compressivestrength measurement among the three dimensions of the structure. A CSBof 1.2-1.4 or lower is desirable for foam structures used in floral andcraft applications.

EXAMPLE 2

Additives were: HBCD at 0.85 pph, LLDPE at 0.30 pph, magnesium oxide at0.04 pph, calcium stearate at 0.08 pph, and talc at 0.025 pph. Theblowing agent was a mixture of 5.0 pph HFC-152a, 2.25 pph ethylchloride, and 1.2 pph carbon dioxide. The die gap was 2.0 mm, the diepressure was 5200 kPa and the forming temperature was 125.6° C.

The foam structure produced had a cell size of 0.44 mm, a desirablelarge average cell size. The structure further had a density of 30.3kg/m³, and a heat distortion temperature (HDT) of 185° C. The HDT is thetemperature at or above which the foam structure will distort.Preferably, the HDT is as high as possible. This foam structure isparticularly desirable for signboard and other graphic art applicationsbecause of its high heat distortion.

EXAMPLE 3

Additives were: HBCD at 0.75 pph, barium stearate at 0.05 pph,tetrasodium pyrophosphate at 0.05 pph, and LLDPE at 0.30 pph. Theblowing agent was a mixture of 6.6 pph HFC-152a and 2.85 pph ethylalcohol. The die gap was 2.9 mm and the die pressure was 4082 kPa.

The foam structure produced had a cell size of 1.0 mm, a desirable largeaverage cell size. The structure further had a density of 30.1 kg/m³, acompressive balance of 1.2, and a HDT of 185° C.

While embodiments of the process and the foamable composition of thepresent invention have been shown with regard to specific details, itwill be appreciated that depending upon the manufacturing process andthe manufacturer's desires, the present invention may be modified byvarious changes while still being fairly within the scope of the novelteachings and principles herein set forth.

What is claimed is:
 1. The process for making an extruded, closed-cellalkenyl aromatic polymer foam structure having an average cell size offrom about 0.3 to about 3.0 millimeters, comprising:a) heating analkenyl aromatic polymer material comprising greater than 50 percent byweight alkenyl aromatic monomeric units to form a melt polymer material;b) incorporating into the melt polymer material at an elevated pressurea blowing agent comprising as a primary blowing agent greater than 50percent by weight 1,1-difluoroethane and from about 10 to less than 50percent by weight of a secondary blowing agent based upon the totalweight of the blowing agent to form a foamable gel, the secondaryblowing agent being more soluble in the melt polymer than1,1-difluoroethane is in the melt polymer, the secondary blowing agentfurther having a lower vapor pressure in air at 25° C. than1,1-difluoroethane; c) cooling the foamable gel to an optimum foamingtemperature prior to extrusion through a die; and d) extruding thefoamable gel through the die into a region of lower pressure to form thefoam structure.
 2. The process of claim 1, wherein a nucleating agent isincorporated into the melt polymer material.
 3. The process of claim 1,wherein the blowing agent is incorporated into the melt polymer materialat a concentration of from about 0.2 to about 5.0 moles per kilogram ofmelt polymer material.
 4. The process of claim 1, wherein the meltpolymer material comprises greater than 90 percent by weight alkenylaromatic monomeric units.
 5. The process of claim 1, wherein the alkenylaromatic polymer is polystyrene.
 6. The process of claim 1, wherein thesecondary blowing agent has a vapor pressure in air at 25° C. of lessthan 580 kilopascals and a solubility in polystyrene of 200,000 weightaverage molecular weight of greater than 1.9 parts per hundred at 25° C.per atmosphere pressure based upon the weight of the polymer.
 7. Theprocess of claim 1, wherein the blowing agent further comprises lessthan 15 weight percent based upon the total weight of the blowing agentof a tertiary blowing agent different than the primary or secondaryblowing agent.
 8. The process of claim 7, wherein the tertiary blowingagent is one or more inorganic blowing agents.